1. Field of the Invention
The present invention relates to a reduction radiation exposure apparatus used in a semiconductor manufacturing apparatus and using a reflection type mask, and to a method of manufacturing a semiconductor element using the reduction radiation exposure apparatus.
2. Description of the Related Art
In recent years, in order to increase the degree of integration and operation speed of a solid device in an LSI or the like, micropatterning of the circuit pattern has advanced. In circuit pattern formation in the process of manufacturing such an LSI, a reduction projection exposure apparatus using vacuum ultraviolet rays as an exposure light is popularly used. In this case, since the resolution is dependent on a wavelength .lambda. of an exposure light and a numerical aperture NA of a projection optical system, a resolution limit is improved by increasing the numerical aperture NA. However, because of a decrease in the depth of focus (D.O.F.) and the difficulty of refraction optical system designing/manufacturing techniques, the resolution limit is almost close to the lower limit. For this reason, the exposure wavelength .lambda. is shortened to improve the resolution. For example, the light source shifts to i-lines (.lambda.=365 nm) and a KrF excimer laser (.lambda.=248 nm). However, because of a theoretical limit depending on a wavelength, even if the conventional exposure technique is improved, a resolution of 0.1 .mu.m or less cannot be easily obtained.
As an exposure technique in the future, a technique for forming a micropattern using high-intensity X-rays from an SR light source or the like has been proposed. X-ray exposure methods are roughly classified into a proximity X-ray exposure method using soft X-rays having a wavelength of 0.5 nm to 2 nm and a reduction projection exposure method using soft X-rays and a reflection type mask. In the former, the present applicant proposed an exposure apparatus described in Japanese Unexamined Patent Publication No. 2-100311. In this method, since an exposure wavelength is short, a high resolution of 0.1 .mu.m or less may be theoretically obtained. In the proximity X-ray exposure method, a transmission type mask is used. A portion which is allowed to transmit X-rays in the proximity X-ray mask is generally constituted by a 35 mm.times.35 mm thin film which consists of a light-element material such as SiN.multidot.SiC, called a membrane, having a thickness of about 2 .mu.m. As a portion which absorbs X-rays in the proximity X-ray mask, a circuit pattern called an absorber and consisting of a heavy metal such as W, Au, or Ta and having a thickness of about 0.5 to 1.5 .mu.m is formed on the membrane. In the proximity X-ray mask, since the circuit pattern is formed on the membrane having very low rigidity, the following problems are pointed out. That is, the circuit pattern is distorted by stress of the absorber or an external force or the like generated when the X-ray mask is placed in a predetermined exposure apparatus, so that a desired circuit pattern cannot be transferred to the resist on the wafer. In particular, in the proximity X-ray exposure method, since the pattern of the proximity X-ray mask is transferred to the resist in a one-to-one magnification, the pattern distortion on the proximity X-ray mask is transferred to the resist at a one-to-one magnification. For this reason, an amount of distortion allowed in the X-ray mask is very small, and a technique for forming the X-ray mask is a problem.
On the other hand, an X-ray reduction projection exposure method of the later using vacuum ultraviolet rays or soft X-rays as an exposure light and using a reflection type mask has attracted attention. This is because the reflection type mask can be manufactured easier than the proximity X-ray mask and can obtain image performance having high resolution power. FIG. 1 shows an exposure optical system according to an X-ray reduction projection exposure method represented by one described in Japanese Unexamined Patent Publication No. 4-225215 or the like. The vacuum ultraviolet rays or soft X-rays are emitted from an undulator light source 101, reflected from a convex total reflection mirror 102, reflected from a concave multi-layered reflection mirror 103, and then illuminate the reflection type mask 104. A multi-layered film which can almost regularly reflect the vacuum ultraviolet rays or soft X-rays is formed on the reflection type mask 104. A predetermined pattern consisting of an X-ray absorber is formed on the multi-layered film. The vacuum ultraviolet rays or soft X-rays reflected by the reflection type mask reach the wafer 106 through reduction projection optical system mirrors 105a to 105d to form the image of a predetermined pattern.
In order to align the reflection type mask 104 and the wafer 106 or enlarge the illumination area of the reflection type mask 104 and the exposure area of the wafer 106 in the optical system shown in FIG. 1, a mask scan stage 107 and a wafer scan stage 108 are used to synchronously scan the reflection type mask 104 and the wafer 106. Since the wavelength of the vacuum ultraviolet rays or soft X-rays used in exposure and illumination ranges from about 5 nm to 20 nm, a theoretical resolution power depending on the wavelength of exposure light is improved.
The present applicant proposed, in Japanese Unexamined Patent Publication No. 1-175731, an exposure apparatus which uses a reflection type mask having a curved shape to correct curvature of the field of the projection optical system. In this case, the shape of the reflection type mask 104 shown in FIG. 1 is a concave curved shape. In this proportion, image performance such as curving of an image surface and astigmatism caused by an X-ray reflection mirror used in X-ray reduction projection cannot be completely corrected by the X-ray reflection mirror shape. When the X-ray mask to be used is constituted by a curved shape such as a spherical shape, curvature of field or astigmatism on a projection surface can be corrected to improve image performance, thereby obtaining preferable optical characteristics.
In the exposure apparatus using the X-ray reduction optical system shown in FIG. 1, in the description of the prior art, because of improvement of resolution and an increase in transfer area, a scan exposure scheme in which a stage has a mask formed thereon, and a mask scan stage moves synchronously with a wafer scan stage is mainly used. However, when a curved mask described in Japanese Unexamined Patent Publication No. 1-175731 to correct curvature of field or astigmatism in the scan exposure scheme, out-of-focus occurs in the transfer surface in relation to a D.O.F. (Depth Of Focus), and preferable pattern projection cannot be achieved over the entire area. This is disadvantageous.